ORIGINS OF EMBRYONIC PATTERNS 687 



to the region in contact or elsewhere as desired and then brought into 

 water. The eggs remain in the obHquely inverted position for some 15 

 minutes until appearance of perivitelline fluid permits orientation to grav- 

 ity by partial rotation, the basal pole rotating downward, the apical pole 

 upward, in a vertical plane passing through the polar axis ("rotation of 

 orientation") ; that is, the heavier basal pole passes downward in the direc- 

 tion of its inclination from the vertical. In this orientation there is no 

 rotation about the polar axis. The later 30° rotation of fertilization is in 

 the opposite direction. In 87 of 100 of these eggs the median plane is 

 within 45° of the plane of rotation of orientation, in 9 eggs it is between 

 45° and 90°. The gray crescent forms on the side of the egg above the 

 basal pole as the rotation of orientation begins, and this side becomes 

 dorsal. The same results are obtained with electrically activated, as 

 with fertilized, eggs and with activation before or after orientation. In 

 eggs inseminated locally at the equator with basal pole directly upward 

 the angle between median plane and entrance point of sperm is less than 

 45° in 147 of 150 eggs and less than 15° in 78 of these. In these eggs the 

 sperm is a more or less effective factor in determining dorsiventraUty. 

 When sperm and rotation of orientation act in different directions, the 

 latter overcomes the former. The localization of dorsal regions and median 

 planes by change in position of cytoplasm and yolk by reaction to gravity 

 in eggs maintained in inverted and partly inverted position has already 

 been discussed (pp. 428-30). In general, the data indicate a physiological 

 basis for dorsiventrality in the unfertilized egg, but this is evidently al- 

 terable experimentally before the superficial movements associated with 

 formation of the gray crescent take place. 



DORSIVENTRALITY IN OTHER VERTEBRATES 



In the meroblastic eggs of fishes, reptiles, and birds the longitudinal 

 or polar axis of the embryo may probably be regarded as coinciding with 

 an egg meridian, though it is practically at right angles to the egg axis. 

 The dorsal side of the embryo is the side adjoining the free surface of the 

 blastoderm; consequently, the question of symmetry becomes the ques- 

 tion of how embryonic polarity is determined. A possible indication of 

 axiate pattern and symmetry, consisting in a regional difference in the 

 marginal periblast, was described by Ruckert (1892). One side of the 

 teleost blastodisc, supposedly the posterior side, is thicker than the other 

 before cleavage.^" Also, one side becomes more susceptible in early stages 



3° Oellacher, 1872; Agassiz and Whitman, 1885; Kowalewsky, 1886. 



